Two-frequency data signals, often referred to as "biphase" or "F/2F" data signals, are widely used for encoding data on magnetic media such as a magnetic stripe typically found on cards, passbooks and other tokens. Magnetic stripe bearing cards or passbooks are typically employed with an automatic teller machine or point of sale terminal or other types of terminals requiring magnetic stripe reading. The magnetic stripe typically includes data concerning a user's bank, account number and other financial data.
Two-frequency data signals are typically encoded on magnetic media as a series of flux reversals. These flux reversals provide a clocking transition and a data transition. The linear distance between two clocking transitions on a stripe is called a "bit cell". When the bit cell contains no intermediate data transition or flux reversal the bit cell is assigned a binary value of `0`. When the bit cell includes one flux reversal or data transition between the clocking transitions within the cell, the cell is assigned a binary value of `1`. The data transition in a `1` bit cell is typically placed at or near the mid point of the bit cell.
FIG. 1 illustrates an example of a two-frequency data signal. It may be seen that a `0` bit may be represented by a bit cell having first and second signal peaks of opposite polarity at the beginning and end thereof respectively (i.e., one peak-to-peak displacement), and a "1" bit may be represented by a bit cell having signal peaks of same polarity at the beginning and end thereof and a signal peak of opposite polarity therebetween (i.e., two peak-to-peak displacements). Accordingly, it may be seen that if the `0` bit is assumed to be encoded at a frequency F, the `1` bit has a frequency of 2F; hence the F/2F designation for these data signals.
F/2F data signals are typically read using a magnetic read head and decoder circuitry to identify zero crossing transitions. An example of such a decoder is illustrated in U.S. Pat. No. 4,254,441 to Fisher which discloses a magnetic read head and a decoder which uses amplitude discrimination and zero slope detection to detect `0` and `1` bits. Unfortunately, zero crossing transitions tend to become obscured by noise as the signal degrades, thereby making accurate decoding difficult.
Since the clocking of F/2F signals is dependent upon a constant rate of relative movement between the magnetic stripe and the read head, much development effort has been directed to providing constant speed heads and/or card transports. See for example U.S. Pat. No. 4,593,328 to Bause, Jr. which discloses a timing wheel which frictionally engages a card as it is moved past a reader to provide timing information.
A major problem in decoding two-frequency signals is decoding a degraded signal. It is well known that magnetic media signals degrade over time. Such degradation may result in a reduction of the amplitude of the flux reversals due to magnetic flux dissipation processes. Degradation may also occur as a result of the introduction of noise signals, in the form of spurious flux reversals or perturbations, in the magnetic media due to stray magnetic fields. Moreover, the physical magnetic media itself is subject to physical abrasion which may remove whole sections of the magnetic media and create fractures in the media.
Typical F/2F data encoded on magnetic media includes a parity bit and a longitudinal record code (LRC) which may be employed to verify the integrity of the F/2F data. Cards which fail these integrity tests, and cards which are unreadable due to a degraded signal, are rejected by the automatic teller machine or point of sale terminal. The intended transaction is not completed, and the card holder is told to obtain a new card.
Rejected cards are a problem for the card holder and the card issuer. Rejected cards are a source of frustration for the card holder, because he cannot use the card in automatic teller machine or point of sale transactions. The card holder must contact the card issuer to obtain a new card. Rejected cards are also a problem for the issuer because consumers will tend not to use the card until a new card is issued. Rejected cards are also a problem for automatic teller machine and point of sale terminal manufacturers because users often blame a rejected card on a problem in the automatic teller machine or point of sale terminal. Often, service calls are made for the automatic teller machine or point of sale terminal when in fact the card itself is defective.
Card issuers have attempted to minimize the number of rejected cards by periodically replacing all cards. However this replacement process is extremely costly. Moreover, when all cards are replaced after a predetermined time, some of the cards will have degraded before the replacement time, and other cards which are replaced have not yet degraded. Accordingly, periodic replacement is not a satisfactory solution.
The magnetic stripe reader art has also attempted to minimize the number of rejected cards by improving the signal decoders to enhance decoding of degraded signals. For example, the above mentioned U.S. Pat. No. 4,254,441 to Fisher discloses a system for reading two-frequency data which employs a read head having an unusually large read aperture and an analog amplitude discrimination and zero slope detection circuit for reading `0` and `1` bits respectively. Another attempt to decode degraded F/2F signals is described in U.S. Pat. No. 4,626,670 to Miller, in which the bit cell width is tracked dynamically to compensate for bit spreading and other degradations which cause the bit cell width to vary. In the Miller patent, the current bit cell width is set as the average of the most recently decoded bit cell widths (for example, the last two bit cells). Unfortunately, this technique has been found to inadequately compensate for bit cell spreading that occurs near a fracture of the magnetic media.
The art has also attempted to read degraded data on cards by performing second and subsequent reads of the card if a first read fails. Unfortunately, each subsequent read attempt on a degraded card further degrades the data. Multiple attempts to read the card further aggravates the signal degradation problem. Moreover, it has been found that often only a portion of the two-frequency data on the card is unreadable, due to a noise pulse, media fracture or other occurrence at a localized portion of the media. However, prior art readers have been unable to regain synchronization of the two-frequency signal after the unreadable portion, so that the entire signal becomes unreadable even though only a small portion is degraded.